1. Field of the Invention
The present invention relates to the improvement of conventional combustors by applying a tube-nested combustion system, which has a remarkable effect when applied to the combustion chamber of an ordinary boiler, to other types of combustors, in particular to those of an ordinary gas turbine system. The invention also relates to a great expansion of the field application of gas turbine equipment with such an improvement.
2. Description of the Prior Art
The features of combustors of conventional gas turbines are as follows. A gas turbine system is now widely used for electric generation and for industrial uses, or for a gas turbine - steam turbine combined cycle, or so-called gas turbine cogeneration. There are the following problems common to all of these gas turbine systems. The first problem is that, since the higher the gas temperature is at the inlet, the higher the thermal efficiency, as indicated by the so-called Carnot's theoretical efficiency, the temperature of the inlet gas which flows into the space around the blades of the gas turbine should be raised so far as is practical. In the present situation, however, for example, the gas temperatures around the turbine blades and the gas turbine inlet are controlled to 990.degree.-1,100.degree. C., so as to reduce the heat resistance as well as the heat loss due to the raised temperature of each part of the turbine blades and the combustor. Indeed, if complete combustion were made in the vicinity of the stoichiometric air ratio, the combustion temperature of the so-called fossil fuel would be on the order of 2,130.degree. C. In practice, however, the combustor and the turbine blades will not serve their practical ends unless their temperatures are lowered. On this account, of the air supplied into the combustor, the air which participates in the actual combustion, or the so-called combustion air, is only about 35-40% of the total. Another about 20% is charged in as dilution air for reducing the temperature of the combustion gas, and the balance of 40% is fed in as the cooling air for the prevention of damage by fire to each part of the combustor and the turbine blades, etc. This represents the bench mark of the present technology.
The aforementioned first problem is quite an important problem. The temperature of the combustion gas, which has once been elevated, is lowered with the dilution air. On the other hand, the cooling air is used for raising the heat resistive temperatures of the combustor and the turbine blades, etc. This involves, besides the large exergy (irreversible) loss due to the cooling of combustion gas with the dilution air, decrease of turbine work resulting from part of air that does not serve as the cooling air, aerodynamic characteristic loss caused by blowing the cooling air out through the blade surface and the walls of flow paths of the combustor, the turbine, and the like, pumping loss of the moving blade cooling air, the loss due to the fact that the cooling air comes to mix with the main stream gas so that the main stream gas is cooled, and so on. Further, there occurs an increase in the pressure loss resulting from an increased amount of combustion gas or a large exergy loss. The sum of all these losses well exceeds about 10%, as converted to the turbine work. Such a loss greatly offsets the benefits in the output and the thermal efficiency which otherwise would be expected to accrue from the raised gas temperature. What is more important, in the present situation heat recovery is made later from the combustion exhaust gas, swollen by a large amount of dilution air and cooling air as described above, which can be said to actually be a serious matter, as compared with normal boilers, in which the combustion is made with an exhaust gas containing 1-3% O.sub.2, 3-5 times as large an amount of exhaust gas is required. As a result, 3-5 times as large a volume is required for all equipment located downstream, including a waste heat boiler, ducts, de-NO.sub.x equipment and chimneys, etc., resulting in a marked economical disadvantage. Conversely, any ingenious contrivance could possibly reduce the combined cycle or cogeneration in the present situation to less than 1/3-1/5 in volume.
A more important fact is that as the gas temperature at the turbine blade inlet goes up above 1,300.degree. C., the necessary volume of cooling air needs to be drastically cut down from the present level. Accordingly, not only some innovation in the technique for cooling turbine blades, etc., is desired, but other techniques corresponding to this one, such as water cooling or steam cooling, are necessary.
The second problem is that of heat loss. Since about 3-5 times as large an amount of dilution air and cooling air as that of the combustion air of the normal boiler is charged in for reducing the combustion gas temperature, as described above, the amount of the exhaust gas increases 3-5 fold. Consequently, elements of the equipment should be increased to 3-5 times in size, with a correspondingly large heat radiation loss, and besides, the heat loss accounted for by the exhaust gas itself runs 3-5 times as large. Thus, conversely, such heat loss and exhaust gas loss could possibly be reduced to 1/3-1/5 of that in the present state by some ingeneous means. As described above, the oxygen (O.sub.2) concentration % in the exhaust gas from a gas turbine reaches 15-16%. In the present state, since the combustion temperature in the combustor is high, the concentration of NO.sub.x generated is high, being 95-240 ppm (O.sub.2 =16%) or 400-1,000 ppm, as converted to that at 0% O.sub.2, in normal city gas (13A) under the present unsuppressed state. How large this amount should be regarded may be discerned from the fact that the boiler's NO.sub.x concentration is regulated to be 60-200 ppm, as converted to that at 0% O.sub.2.
As a third problem, in a gas turbine plant (the so-called combined cycle or cogeneration) of this type, particularly, hereafter attachment of large de-NO.sub.x equipment will be indispensable, making problems involving its cost, dimensions and space, etc., important. These problems will probably increase in importance from now on because of the growing demand of users for elevating the gas turbine inlet temperature.
Where a fourth problem is concerned, in a conventional gas and steam turbine combined cycle, or cogeneration, O.sub.2 in the exhaust gas is as high as about 15-16%. Its discharge is uneconomical. Accordingly, there has been taken a measure in which a duct burner is placed on the upstream side of the waste heat boiler and fuel is fed therein, to be burned, using the residual O.sub.2 in the exhaust gas, to raise the combustion gas temperature, thereby increasing the amount of steam recovered from the waste heat boiler. According to this system, however, a large duct burner and a long duct as its burner chamber equivalent are required. Besides, the potential temperature rise is limited by the factors of the heat resistance of these parts the and that heat loss and the outlet O.sub.2 concentration can be reduced to only about 10%. Thus in the final analysis, the exhaust gas must be released at a high O.sub.2 %, as it is, without recovering the exhaust gas heat loss.